Choice D is the best answer. This question tests dash usage for dramatic revelation.

1. Context clues: The identity is dramatically revealed as surprising.
2. Meaning: A dash creates dramatic effect when revealing surprising information.
3. Verify: "The detective finally identified the culprit—her own trusted colleague" creates dramatic tension.

💡 Strategy: Use a dash for dramatic effect when revealing surprising or unexpected information.

Choice A is incorrect because a comma doesn't provide dramatic effect. Choice B is incorrect because a semicolon requires independent clauses. Choice C is incorrect because while a colon works, a dash is more dramatic here.